Cables with concentric, coaxial ground-returns and high-resistivity grading layers are by far the preferred means of underground power distribution at primary voltage in North America. Often, when it is necessary to open such a cable between endpoints for purposes of splicing or testing, that cable may be one among several identical sets of cables at the site of operations. Positive identification of the correct set of cables is difficult, and even after identification, one cannot ascertain from the safe exterior of the cable whether it is live or dead. Mistakes in attempts at identification or in determining the cable status can be costly and dangerous; at best they leave a cable in need of repair and at worst they represent a considerable hazard to personnel. To avoid such hazards, methods have been developed to get "inside" the cable without subjecting the maintenance personnel to undue risk, but these methods are either lengthy (slow and meticulous peeling back of the several layers of sheath and conductor to reach the primary insulation) or potentially destructive of equipment and service (chopping the cable from a safe distance using a remote cable cutter). Often such work has to be done under difficult conditions of weather or in the wet and claustrophobic interior of a manhole. Speed, ease of use and reliability of equipment for the work are all critically important because they both lower cost and increase safety.
The same problems of identification and determination of status were encountered with the lead-sheathed cable which is still used in older urban underground distribution systems. Solutions to those problems are disclosed in U.S. Pat. No. 4,760,327, which is assigned to the assignee of this application. A small, easily repaired hole was formed by removing a plug of lead from the sheath to expose the grading or screen layer. Then, two probes were placed in contact with the high-resistivity grading or screen layer and the sheath, respectively. The line-frequency voltage drop caused by the flow of displacement current from the screen layer to the lead sheath was then observed to determine whether the cable was live or dead. These solutions were adequate but left room for improvement, especially in the area of detection of signals against a background of noise which is often quite high.
In addition to the improvement of methods associated with lead-sheathed cable, the need for accurate status determination in plastic-sheathed, concentric-wire-shield cable was also recognized. The principal variations in the lead cable are diameter and the resistivity of the screen layer. The sheet resistances of the screen layers vary by several orders of magnitude among cables of generally identical specification, while the lead thickness is reasonably constant over cables of very different ampacity. Concentric-wire-shield cables from different manufacturers show considerably more variation, with no agreement on the handedness of the helical shield, on the number or size of the shield wires therein, nor even on whether the wires should be regularly spaced or wound rather randomly onto the cable. These cables also show the same high variability in the screen-layer resistivity as the lead-sheathed cable. As noted above, with lead cable, contact with the continuous lead sheath is relatively easily achieved and removal of a lead plug permits probe contact with the screen layer. With concentric-wire-shield cables, to contact the shield wire one must probe in the outer plastic insulation, and find the shield wires without damage to the mechanical or electrical integrity of the primary insulation. Good contact must be obtained between a shield wire and at least one probe as well as between the screen layer and another probe to establish a test circuit. Finally, the port that is opened to gain access to the cable interior must be recloseable in some relatively simple way, which restores the watertight integrity of the outer jacket. All this must be accomplished safely, quickly, reliably, and without taking the cable out of service.
The primary object of the present invention is achieving improvement of techniques and equipment for the safe and accurate determination of the energization status of cables.
Other objects are to assure that under no circumstances will a false negative on cable energization status be given, despite the number of events which could cause false negative to appear. The electronics employed must detect and infallibly report any problem rather than give an erroneous or misleading reading of cable status.
Also, the electronics should have the capacity to adapt itself to wide ranges of resistivity of the screen layer and should self-test and cable-test on a continuous recycling basis to assure that the cable is dead at the precise time it is actually cut.
Finally, the apparatus must be portable, rugged and weather-resistant to survive truck travel, inclement weather, and dripping manholes.